[单选题]
A distinctive feature of qwwo.q2-xvan:edn / 4zuayey fs/7w; 0hjsps2 1+8te-alcmheb-rks is that they
A. have zero rest mass.
B. have zero charge.
C. have fractional electric charge.
D. are always observed singly, since they do not readily interact with other particles.
